<p>The author compares the photometric effects of motion (which is defined as the variation of a point's imaged intensity as a consequence of motion) and the geometric effects of motion (which is defined as the variation in projected surface geometry as a consequence of motion). It is shown that photometric motion provides a cue to surface shape that is potentially as useful as that provided by geometric motion. A simple technique for using this photometric motion information to extract both surface shape and reflectance is developed, and a biological implementation is proposed. How this photometric motion mechanism can be integrated with and used to enhance existing structure-motion algorithms is discussed. Intensity information is sometimes even more important than geometric distortion when estimating the shape of a single, continuous surface that is rotating relative to the observer's frame of reference.</p>